This invention relates to a tubular burner, and more specifically to a tubular hollow burner of desired cross-section being axially long compared to the dimensions of the cross-section. The invention also relates to a distributor disposed internally of the hollow burner which ensures good mixing of the gas and air in the combustible fluid urged through the burner and as near to an even flame profile as possible on the flame strip of said burner.
Although the following description related almost exclusively to fully pre-mixed cylindrical tubular burners, it is to be mentioned that the burner of the present invention may be used for non pre-mixed and normally aspirated applications and furthermore the cross-sectional shape of the burner is unimportant The invention should accordingly not be considered as limited by the following description. It should also be mentioned that the word tubular as used herein describing burners is to be taken to mean any burner having an outer surface which defines an internal cavity in which distribution means can be disposed.
Pre-mixed burners are so-called because the fuel, usually gas (under denominations for reference gases and test gases identified in European Standard EN 297), and a fan supplied quantity of air exceeding the stoichiometrically correct amount of air for the specific gas type (superstoichiometric) are mixed to produce a combustible mixture which subsequently is passed through the burner and ignited to produce a burner flame that, in the case of heating the water in a boiler, is applied to a heat exchanger of the boiler. The term pre-mixed arises therefore because of the mixing of the fuel and air before the combustible mixture passes through the flame strip.
There are other types of burners which operate in a mode in which a sub-stoichiometric amount of primary combustion air is mixed with the fuel before the flame strip, secondary air required for completing the combustion process being induced into the flame after ignition of the gas/primary air mixture. These other burners are known as partially pre-mixed burners. The present invention may be applicable to such burners, but its best application is to the fully pre-mixed type, as partially premixed burners are limited by the relatively high levels of nitrogen oxides (NOx) they generate during the combustion process and as such, these burners are diminishing in popularity.
Fully pre-mixed burners tend to be high intensity burners in which high volumes of gas/air mixture are forced through a relatively small area (in plan) burner, and specifically through the ports in the flame strip to give a compact, high intensity flame which sits on or near the surface of said flame strip.
The high volume of the gas/air flow being urged through the small port area of the burner flame strip means there is provided a high xe2x80x9cport-loadingxe2x80x9d on each individual burner port. The fact that a compressible mixture flows through the burner at a certain velocity means that any instabilities created on ignition of said mixture are amplified and can ultimately develop a common frequency which constructively harmonizes with the natural frequency of the boiler system to generate a phenomenon called combustion resonance which is manifested in noise. Noise of any audible volume or frequency is unacceptable for pre-mixed burner applications.
The boiler system comprises the combustion chamber, the heat exchanger which will occupy a predetermined position within said combustion chamber, and a flue attached to said chamber to vent the exhaust gases of combustion. Any variance of these parameters will influence the harmonics of the system e.g. varying the flue length will change the back pressure on the combustion chamber.
The combustion resonance is manifested as three distinct types of resonance:
1. A low frequency (125 to 200 Hz) rumble on ignition; This is believed to be due to flame instabilities caused by poor gas/air mixing, bad gas/air mixture distribution and poorly timed ignition, such being associated with the burner appliance design factors of upstream mixing of gas and air, position of igniter etc.
2. A higher frequency (250 to 315 Hz) resonance on ignition at volumes up to 95 dB; Under standard repeat ignition conditions the flame ignites and thermally fluctuates initially as it stabilizes near the port. The differential pressures and temperatures created initially in the system exacerbate this instability creating a range of oscillating and fluctuating frequencies of flame vibration, some of which may harmonize and thus be amplified at one or more of the natural resonance frequencies bands of the system. However, once the system has been operational for approximately a minute, these instabilities dissipate and the resonance fades out.
3. A continuous high frequency resonance can develop once the flames have stabilized. This can arise from instabilities caused by ignition resonance and which are continuously excited by virtue of the gas/air flow movements within the system during operation, or by the inherent excitations developed by virtue of the burner design.
Currently, conventional metal burners of the type considered herein having ports integrally formed on an outer surface thereof cannot offer the xe2x80x9cturndownxe2x80x9d range (the range over which the flame is stable on or proximate the surface of the burner as the gas/air flow is gradually reduced) demanded by manufacturers of modern appliances, and therefore ceramic materials are often used in such applications. Additionally, the risk of xe2x80x9cflashbackxe2x80x9d at low port loadings where the flame burns on or very close to the flame strip must be eliminated and this has heretofore proved difficult.
At much higher port loadings, the flame can lift away from the flame strip as a result of the increased and rapid volumetric throughput of gas through the ports, and flame instabilities can thus result.
The stability of the flame on the burner flame strip is dependent on the open area of said flame strip (port area per unit area of the flame strip), the surface area xe2x80x9clandxe2x80x9d surrounding each port, i.e. the length of the ports, the pattern of ports, the profile of the flame strip surface, and the efficacy of any distribution means disposed behind the flame strip internally of the burner. A stable flame requires sufficient xe2x80x9clandxe2x80x9d to anchor itself thermally to the port, but this requirement compromises the total open area of the plaque and hence increases the port loading, with the attendant disadvantages of lift-off mentioned above. There is therefore a trade off between these two parameters.
A further consideration in modern burner design is the recently introduced requirements for reducing emissions of noxious gases such as NOx (Nitrogen Oxide) and CO (Carbon Monoxide) below predetermined limits.
U.S. Pat. No. 5,743,727 to Rodgers describes a burner of similar construction to that with which the invention is concerned. Specifically, a cylindrical tubular burner is described having an outer cylindrical tube body with a blind or capped end and a plurality of perforations provided over a small sector of the body along the length of said tube. This perforated sector forms the flame strip of the burner and internal distribution of the combustible gas mixture underneath the flame strip to obtain an even flame profile thereon is achieved by means of an elongate distributor of similar cross-sectional shape but of smaller size so that the distributor can be slid within the body and sealed to the blind end or cap thereof.
Injection means is provided at the open end of the distributor and air may be drawn thereinto along with the gas so that a combustible mixture (which may at that stage not be evenly or thoroughly mixed) passes through the hollow distributor. In accordance with the invention of the U.S. patent, a plurality of outlets are provided on the underside of the distributor approximately 180xc2x0 from the sector over which the perforations on the burner body are provided so that the combustible gas is mixed thoroughly with air before being urged through said perforations. Furthermore, the distributor outlets are preferably disposed only towards the end of the distributor which is sealingly affixed to said body blind end or cap so that some initial mixing of the combustible gas and air occurs in the front end of the distributor before these gases pass through the outlets and flow circumferentially around the outer surface of the distributor upwardly within the body and ultimately through the perforations in the upper sector of said body.
The effective distribution of the combustible gas/air mixture is achieved by disposing the outlets towards the far end of the distributor proximate the blind end of the body and providing louvers on the outlets internally of the distributor so that the gaseous mixture emerging from said outlets not only travels circumferentially around the distributor towards the flame strip but also in a reverse direction to that in which the mixture travels within the distributor. The result is a reasonably even gas pressure behind the entire flame strip and thus a substantially uniform flame profile.
Although this type of burner configuration achieves reasonable distribution of combustible gas mixture behind the flame strip and thus a generally even flame profile is achieved on the burner flamestrip, the construction and assembly of the burner is expensive, as is the manufacture of the various components involved in said construction.
Additionally this burner suffers from a high static pressure within the distributor tube and additionally immediately behind the flame strip. As a result of this high pressure, the gas mixture and flame distribution is difficult to control and this can compromise the resulting flame profile.
It is an object of this invention to provide a tubular burner having a wide operating range and being efficient from the point of view of substantially reducing noxious gas emissions which is nevertheless simple and Inexpensive to manufacture, construct and assemble.
It is a further object of this invention to provide a burner which provides a good distribution and an even pressure of combustible gas mixture behind the burner flame strip so that a uniform flame profile is obtained on the surface thereof.
It is a yet further object of this invention to provide a burner which provides a solution to the problems associated with the high static pressures developed inside the distribution tube and the hollow burner body.
According to the invention there is provided a tubular burner comprising a tubular body of desired cross sectional shape and being axially long in comparison to the dimensions of said cross-section, said body having an outer surface over an axially aligned portion of which is provided a plurality of perforations which portion constitutes the flame strip of said burner immediately beneath which an upper region of the burner is partially defined internally of said tubular body, said burner further comprising an elongate distributor of substantially the same axial length as the tubular burner body and being of smaller cross-sectional dimension than said body allowing for sliding fitment of said distributor therein, said distributor having a first tubular portion opening proximate an open end of said burner body into which combustible fluid can be introduced, said tubular portion having an upper surface which together with the flame strip defines the upper region of said tubular body, and a second extension portion which is effectively a continuation of the upper surface of said tubular portion and thus also defines the upper region of said body and constrains fluid emerging from the tubular portion to flow therebeneath, characterized in that both tubular portion and extension portion of the distributor are provided with axially-aligned flanges having a predetermined number and distribution of perforations therein, said flanges contacting the inner wails of the burner body to effectively divide the inside of the burner body into an upper chamber behind the flame strip and a lower chamber into which combustible fluid is urged from the tubular portion of said distributor, said fluid passing from the lower chamber into the upper chamber through said perforations and being evenly distributed in said upper chamber by virtue of the number and distribution of said perforations along the length of said distributor.
Most preferably the cross sections of the tubular body and distributor are circular.
Further preferably flanges are provided on diametrically opposite sides of said distributor tubular portion and extension portion or on opposed sides thereof when the cross-section is not circular.
Preferably the tubular burner body is open at either end and the distributor is provided with a cap secured to the free end of said extension portion, said cap having dimensions corresponding to the dimensions of the tubular body to allow for both sliding fitment of said distributor within said body and sealing attachment of said cap to one of said open ends of said body.
Most preferably the distributor consists of a cap portion to which is welded a hemi-cylindrical elongate section having a longitudinal axis substantially coincident with the longitudinal axis of the burner body, said section being orientated so that its zenith is directly beneath an axially parallel line bisecting the flame strip, and a tubular portion secured within the hemi-cylindrical section having a first end coincident with a free end of the hemi-cylindrical section and a second end opening part way along said hemi-cylindrical section.
Preferably the hemi-cylindrical section is welded to the cap and the cap ultimately welded or otherwise secured to said burner body.
The effective division of the burner body by the flanges and the upper hemi-cylindrical section of the distributor into two discreet chambers, one being a mixing chamber and the other being a gas delivery chamber, has a number of advantages.
Firstly, the flanges of the distributor serve as guides therefore during insertion into the burner body.
Secondly, the number and distribution of the perforations in the flanges can be easily altered allowing for simple testing and re-design of the distributor to suit particular circumstances. The perforations also serve as an extremely effective and simple means of distributing the combustible fluid mixture beneath the flame strip.
Thirdly, greater flexibility of air delivery can be achieved, thus imparting a greater degree of control on static pressures depending on the number and disposition of said perforations along the length of the flanges.
Additionally, a greater degree of flexibility can be achieved for mixing of multiple gases.
Fourthly, the distributor offers a common footprint for each burner diameter.